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diff --git a/old/3772-h/files/ch28.html b/old/3772-h/files/ch28.html new file mode 100644 index 0000000..00bc0eb --- /dev/null +++ b/old/3772-h/files/ch28.html @@ -0,0 +1,2138 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> +<!-- saved from url=(0036)http://../Lyell/The Student's Elements of Geology --> +<html> +<head> +<meta name="generator" content="HTML Tidy, see www.w3.org"> +<title>The Student's Elements of Geology: Title</title> +<meta content="text/html; charset=iso-8859-1" http-equiv= +"Content-Type"> +<meta content="MSHTML 5.00.2919.6307" name="GENERATOR"> +<link rel="stylesheet" href="geology.css" type="text/css"> +</head> +<body> +<p><b>The Student’s Elements of Geology</b></p> + +<hr> +<p class="page"><a name="page 494">[ 494 ]</a></p> + +<p> </p> + +<center><b>Chapter XXVIII</b><br> +<br> +VOLCANIC ROCKS.</center> + +<p class="intro">External Form, Structure, and Origin of Volcanic +Mountains. — Cones and Craters. — Hypothesis of +“Elevation Craters” considered. — Trap Rocks. +— Name whence derived. — Minerals most abundant in +Volcanic Rocks. — Table of the Analysis of Minerals in the +Volcanic and Hypogene Rocks. — Similar Minerals in +Meteorites. — Theory of Isomorphism. — Basaltic Rocks. +— Trachytic Rocks. — Special Forms of Structure. +— The columnar and globular Forms. — Trap Dikes and +Veins. — Alteration of Rocks by volcanic Dikes. — +Conversion of Chalk into Marble. — Intrusion of Trap between +Strata. — Relation of trappean Rocks to the Products of +active Volcanoes.</p> + +<p>The aqueous or fossiliferous rocks having now been described, we +have next to examine those which may be called volcanic, in the +most extended sense of that term. In the diagram (Fig. 584) suppose +<i>a, a</i> to represent the crystalline formations, such as the +granitic and metamorphic; <i>b, b</i> the fossiliferous strata; and +<i>c, c</i> the volcanic rocks. These last are sometimes found, as +was explained in the first chapter, breaking through <i>a</i> and +<i>b,</i> sometimes overlying both, and occasionally alternating +with the strata <i>b, b.</i></p> + +<center><img src="../images4/fig584.jpg" width="358" height="105" alt= +"Fig. 584: a. Hypogene formations, stratified and unstratified. b. Aqueous formations. c. Volcanic rocks."> +</center> + +<p><b>External Form, Structure, and Origin of Volcanic +Mountains.</b>—The origin of volcanic cones with +crater-shaped summits has been explained in the “Principles +of Geology” (Chapters 23 to 27), where Vesuvius, Etna, +Santorin, and Barren Island are described. The more ancient +portions of those mountains or islands, formed long before the +times of history, exhibit the same external features and internal +structure which belong to most of the extinct volcanoes of still +higher antiquity; and these last have evidently been due to a +complicated series of operations, varied in kind according to +circumstances; as, for example, whether the accumulation took place +above or below the level of the sea, whether the lava issued from +one or several contiguous vents, and, lastly,</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 495">[ 495 ]</a></p> + +<p>whether the rocks reduced to fusion in the subterranean regions +happened to have contained more or less silica, potash, soda, lime, +iron, and other ingredients. We are best acquainted with the +effects of eruptions above water, or those called subÆrial or +supramarine; yet the products even of these are arranged in so many +ways that their interpretation has given rise to a variety of +contradictory opinions, some of which will have to be considered in +this chapter.</p> + +<center><img src="../images4/fig585.jpg" width="321" height="158" alt= +"Fig. 585: Part of the chain of extinct volcanoes called the Monts Dome, Aurvergne."> +</center> + +<p><i>Cones and Craters.</i>—In regions where the eruption of +volcanic matter has taken place in the open air, and where the +surface has never since been subjected to great aqueous denudation, +cones and craters constitute the most striking peculiarity of this +class of formations. Many hundreds of these cones are seen in +central France, in the ancient provinces of Auvergne, Velay, and +Vivarais, where they observe, for the most part, a linear +arrangement, and form chains of hills. Although none of the +eruptions have happened within the historical era, the streams of +lava may still be traced distinctly descending from many of the +craters, and following the lowest levels of the existing valleys. +The origin of the cone and crater-shaped hill is well understood, +the growth of many having been watched during volcanic eruptions. A +chasm or fissure first opens in the earth, from which great volumes +of steam are evolved. The explosions are so violent as to hurl up +into the air fragments of broken stone, parts of which are shivered +into minute atoms. At the same time melted stone or <i>lava</i> +usually ascends through the chimney or vent by which the gases make +their escape. Although extremely heavy, this lava is forced up by +the expansive power of entangled gaseous fluids, chiefly steam or +aqueous vapour, exactly in the same manner as water is made to boil +over the edge of a vessel when steam has been generated at the +bottom by heat. Large quantities of the lava are also shot up into +the air, where it separates into fragments, and acquires a spongy +texture by the sudden enlargement</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 496">[ 496 ]</a></p> + +<p>of the included gases, and thus forms <i>scoriæ,</i> other +portions being reduced to an impalpable powder or dust. The +showering down of the various ejected materials round the orifice +of eruption gives rise to a conical mound, in which the successive +envelopes of sand and scoriæ form layers, dipping on all +sides from a central axis. In the mean time a hollow, called a <i> +crater,</i> has been kept open in the middle of the mound by the +continued passage upward of steam and other gaseous fluids. The +lava sometimes flows over the edge of the crater, and thus thickens +and strengthens the sides of the cone; but sometimes it breaks down +the cone on one side (see Fig. 585), and often it flows out from a +fissure at the base of the hill, or at some distance from its +base.</p> + +<p>Some geologists had erroneously supposed, from observations made +on recent cones of eruption, that lava which consolidates on steep +slopes is always of a scoriaceous or vesicular structure, and never +of that compact texture which we find in those rocks which are +usually termed “trappean.” Misled by this theory, they +have gone so far as to believe that if melted matter has originally +descended a slope at an angle exceeding four or five degrees, it +never, on cooling, acquires a stony compact texture. Consequently, +whenever they found in a volcanic mountain sheets of stony +materials inclined at angles of from 5° to 20° or even more +than 30°, they thought themselves warranted in assuming that +such rocks had been originally horizontal, or very slightly +inclined, and had acquired their high inclination by subsequent +upheaval. To such dome-shaped mountains with a cavity in the +middle, and with the inclined beds having what was called a +quâquâversal dip or a slope outward on all sides, they +gave the name of “Elevation craters.”</p> + +<p>As the late Leopold Von Buch, the author of this theory, had +selected the Isle of Palma, one of the Canaries, as a typical +illustration of this form of volcanic mountain, I visited that +island in 1854, in company with my friend Mr. Hartung, and I +satisfied myself that it owes its origin to a series of eruptions +of the same nature as those which formed the minor cones, already +alluded to. In some of the more ancient or Miocene volcanic +mountains, such as Mont Dor and Cantal in central France, the mode +of origin by upheaval as above described is attributed to those +dome-shaped masses, whether they possess or not a great central +cavity, as in Palma. Where this cavity is present, it has probably +been due to one or more great explosions similar to that which +destroyed a great part of ancient Vesuvius in the time of Pliny. +Similar paroxysmal catastrophes have caused in historical times</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 497">[ 497 ]</a></p> + +<p>the truncation on a grand scale of some large cones in Java and +elsewhere.*</p> + +<p>Among the objections which may be considered as fatal to Von +Buch’s doctrine of upheaval in these cases, I may state that +a series of volcanic formations extending over an area six or seven +miles in its shortest diameter, as in Palma, could not be +accumulated in the form of lavas, tuffs, and volcanic breccias or +agglomerates without producing a mountain as lofty as that which +they now constitute. But assuming that they were first horizontal, +and then lifted up by a force acting most powerfully in the centre +and tilting the beds on all sides, a central crater having been +formed by explosion or by a chasm opening in the middle, where the +continuity of the rocks was interrupted, we should have a right to +expect that the chief ravines or valleys would open towards the +central cavity, instead of which the rim of the great crater in +Palma and other similar ancient volcanoes is entire for more than +three parts of the whole circumference.</p> + +<p>If dikes are seen in the precipices surrounding such craters or +central cavities, they certainly imply rents which were filled up +with liquid matter. But none of the dislocations producing such +rents can have belonged to the supposed period of terminal and +paroxysmal upheaval, for had a great central crater been already +formed before they originated, or at the time when they took place, +the melted matter, instead of filling the narrow vents, would have +flowed down into the bottom of the cavity, and would have +obliterated it to a certain extent. Making due allowance for the +quantity of matter removed by subaërial denudation in volcanic +mountains of high antiquity, and for the grand explosions which are +known to have caused truncation in active volcanoes, there is no +reason for calling in the violent hypothesis of elevation craters +to explain the structure of such mountains as Teneriffe, the Grand +Canary, Palma, or those of central France, Etna, or Vesuvius, all +of which I have examined. With regard to Etna, I have shown, from +observations made by me in 1857, that modern lavas, several of them +of known date, have formed continuous beds of compact stone even on +slopes of 15, 36, and 38 degrees, and, in the case of the lava of +1852, more than 40 degrees. The thickness of these tabular layers +varies from 1½ foot to 26 feet. And their planes of +stratification are parallel to those of the overlying and +underlying scoriæ which form part of the same +currents.†</p> + +<p><b>Nomenclature of Trappean Rocks.</b>—When geologists +first began to examine attentively the structure of the +northern</p> + +<p class="fnote">* Principles, vol. ii, pp. 56 and 145.<br> +† Memoir on Mount Etna, Phil. Trans., 1858.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 498">[ 498 ]</a></p> + +<p>and western parts of Europe, they were almost entirely ignorant +of the phenomena of existing volcanoes. They found certain rocks, +for the most part without stratification, and of a peculiar mineral +composition, to which they gave different names, such as basalt, +greenstone, porphyry, trap tuff, and amygdaloid. All these, which +were recognised as belonging to one family, were called +“trap” by Bergmann, from <i>trappa,</i> Swedish for a +flight of steps—a name since adopted very generally into the +nomenclature of the science; for it was observed that many rocks of +this class occurred in great tabular masses of unequal extent, so +as to form a succession of terraces or steps. It was also felt that +some general term was indispensable, because these rocks, although +very diversified in form and composition, evidently belonged to one +group, distinguishable from the Plutonic as well as from the +non-volcanic fossiliferous rocks.</p> + +<p>By degrees familiarity with the products of active volcanoes +convinced geologists more and more that they were identical with +the trappean rocks. In every stream of modern lava there is some +variation in character and composition, and even where no important +difference can be recognised in the proportions of silica, alumina, +lime, potash, iron, and other elementary materials, the resulting +materials are often not the same, for reasons which we are as yet +unable to explain. The difference also of the lavas poured out from +the same mountain at two distinct periods, especially in the +quantity of silica which they contain, is often so great as to give +rise to rocks which are regarded as forming distinct families, +although there may be every intermediate gradation between the two +extremes, and although some rocks, forming a transition from the +one class to the other, may often be so abundant as to demand +special names. These species might be multiplied indefinitely, and +I can only afford space to name a few of the principal ones, about +the composition and aspect of which there is the least discordance +of opinion.</p> + +<p><b>Minerals most abundant in Volcanic Rocks.</b>—The +minerals which form the chief constituents of these igneous rocks +are few in number. Next to quartz, which is nearly pure silica or +silicic acid, the most important are those silicates commonly +classed under the several heads of feldspar, mica, hornblende or +augite, and olivine. In Table 28.1, in drawing up which I have +received the able assistance of Mr. David Forbes, the chemical +analysis of these minerals and their varieties is shown, and he has +added the specific gravity of the different mineral species, the +geological application of which in determining the rocks formed by +these minerals will be explained in the sequel (p.504).</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 499">[ 499 ]</a></p> + +<center><i>Analysis of Minerals most abundant in the Volcanic and +Hypogene Rocks.</i></center> + +<br> +<table border="1" cellspacing="0" cellpadding="6" summary= +"Mineral Groups and Analysis." align="center" class="499"> +<tr> +<td align="center" colspan="5">THE QUARTZ GROUP</td> +</tr> + +<tr> +<td align="left" valign="middle">QUARTZ</td> +<td align="right">100·0<br> +2·6</td> +<td align="left">Silica<br> +Specific gravity</td> +</tr> + +<tr> +<td align="left">TRIDYMITE</td> +<td align="right">100·0<br> +2·3</td> +<td align="left">Silica<br> +Specific gravity</td> +</tr> + +<tr> +<td align="center" colspan="5">THE FELDSPAR GROUP</td> +</tr> + +<tr> +<td align="left" valign="middle">ORTHOCLASE.<br> +—— Carisbad, in granite (bulk)</td> +<td align="right" valign="top">65·23<br> +16·26<br> +0·27<br> +nil<br> +trace<br> +nil<br> +14·66<br> +1·45<br> +nil<br> +2·55</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">—— Sanadine, +Drachenfels in trachyte (Rammelsberg)</td> +<td align="right" valign="top">65·87<br> +18·53<br> +nil<br> +nil<br> +0·95<br> +0·30<br> +10·32<br> +3·49<br> +W. 0·44<br> +2·55</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">ALBITE.<br> +—— Arendal, in granite (G. Rose)</td> +<td align="right" valign="top">68·46<br> +19·30<br> +nil<br> +0·28<br> +0·68<br> +nil<br> +nil<br> +11·27<br> +nil<br> +2·61</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">OLIGOCLASE.<br> +—— Ytterby, in granite (Berzelius)</td> +<td align="right" valign="top">61·55<br> +23·80<br> +nil<br> +nil<br> +3·18<br> +0·80<br> +0·38<br> +9·67<br> +nil<br> +2·65</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">—— Teneriffe, in +trachyte (Deville)</td> +<td align="right" valign="top">61·55<br> +22·03<br> +nil<br> +nil<br> +2·81<br> +0·47<br> +3·44<br> +7·74<br> +nil<br> +2·59</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">LABRADORITE.<br> +—— Hitteroe, in Labrador-rock (Waage)</td> +<td align="right" valign="top">51·39<br> +29·42<br> +2·90<br> +nil<br> +9·44<br> +0·37<br> +1·10<br> +5·03<br> +W. 0·71<br> +2·72</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">—— Iceland, in +volcanic (Damour)</td> +<td align="right" valign="top">52·17<br> +29·22<br> +1·90<br> +nil<br> +13·11<br> +nil<br> +nil<br> +3·40<br> +nil<br> +2·71</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">ANORTHITE.<br> +—— Harzburg, in diorite (Streng)</td> +<td align="right" valign="top">45·37<br> +34·81<br> +0·59<br> +nil<br> +16·52<br> +0·83<br> +0·40<br> +1·45<br> +W. 0·87<br> +2·74</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">—— Hecla, in volcanic +(Waltershausen)</td> +<td align="right" valign="top">45·14<br> +32·10<br> +2·03<br> +0·78<br> +18·32<br> +nil<br> +0·22<br> +1·06<br> +nil<br> +2·74</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">LEUCITE.<br> +—— Vesuvius, 1811, in lava (Rammelsberg)</td> +<td align="right" valign="top">56·10<br> +23·22<br> +nil<br> +nil<br> +nil<br> +nil<br> +20·59<br> +0·57<br> +nil<br> +2·48</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">NEPHELINE.<br> +—— Miask, in Miascite (Scheerer)</td> +<td align="right" valign="top">44·30<br> +33·25<br> +0·82<br> +nil<br> +0·32<br> +0·07<br> +5·82<br> +16·02<br> +nil<br> +2·59</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">—— Vesuvius, in +volcanic (Arfvedson)</td> +<td align="right" valign="top">44·11<br> +33·73<br> +nil<br> +nil<br> +nil<br> +nil<br> +nil<br> +20·46<br> +W. 0·62<br> +2·60</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="center" colspan="5">THE MICA GROUP</td> +</tr> + +<tr> +<td align="left" valign="middle">MUSCOVITE.<br> +—— Finland, in grante (Rose)</td> +<td align="right" valign="top">46·36<br> +36·80<br> +4·53<br> +nil<br> +nil<br> +nil<br> +9·22<br> +nil<br> +F. 0·67<br> +W. 1·84<br> +2·90</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> + <br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">LEPIDOLITE.<br> +—— Cornwall, in granite (Regnault)</td> +<td align="right" valign="top">52·40<br> +26·80<br> +nil<br> +1·50<br> +nil<br> +nil<br> +9·14<br> +nil<br> +F. 4·18<br> +Li. 4·85<br> +2·90</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> + <br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">BIOTITE.<br> +—— Bodennais (V. Kobel></td> +<td align="right" valign="top">40·86<br> +15·13<br> +13·00<br> +nil<br> +nil<br> +22·00<br> +8·83<br> +nil<br> +W. 0·44<br> +2·70</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">—— Vesuvius, in +volcanic (Chodnef)</td> +<td align="right" valign="top">40·91<br> +17·71<br> +11·02<br> +nil<br> +0·30<br> +19·04<br> +9·96<br> +nil<br> +nil<br> +2·75</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">PHLOGOPITE.<br> +—— New York, in metamorphic limestone +(Rammelsberg)</td> +<td align="right" valign="top">41·96<br> +13·47<br> +nil<br> +2·67<br> +0·34<br> +27·12<br> +9·37<br> +nil<br> +F. 2·93<br> +W. 0·60<br> +2·81</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> + <br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">MARGARITE.<br> +—— Nexos (Smith)</td> +<td align="right" valign="top">30·02<br> +49·52<br> +1·65<br> +nil<br> +10·82<br> +0·48<br> +1·25<br> + <br> +W. 5·55<br> +2·99</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +=Potash<br> +=Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">RAPIDOLITE.<br> +—— Pyrenees (Delesse)</td> +<td align="right" valign="top">32·10<br> +18·50<br> +nil<br> +0·06<br> +nil<br> +36·70<br> +nil<br> +nil<br> +W. 12·10<br> +2·61</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">TALC.<br> +—— Zillerthal (Delesse)</td> +<td align="right" valign="top">63·00<br> +nil<br> +nil<br> +trace<br> +nil<br> +33·60<br> +nil<br> +nil<br> +W. 3·10<br> +2·78</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="center" colspan="5">THE AMPHIBOLE AND PYROXENE +GROUP</td> +</tr> + +<tr> +<td align="left" valign="middle">TREMOLITE.<br> +—— St. Gothard (Rammelsbeg)</td> +<td align="right" valign="top">58·55<br> +nil<br> +nil<br> +nil<br> +13·90<br> +26·63<br> +nil<br> +nil<br> +F.W. 0·34<br> +2·93</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">ACTINOLITE.<br> +—— Arendal, in granite (Rammelsberg)</td> +<td align="right" valign="top">56·77<br> +0·97<br> +nil<br> +5·88<br> +13·56<br> +21·48<br> +nil<br> +nil<br> +W. 2·20<br> +3·02</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">HORNBLENDE.<br> +—— Faymont, in diorite (Deville)</td> +<td align="right" valign="top">41·99<br> +11·66<br> +nil<br> +22·22<br> +9·55<br> +12·59<br> +nil<br> +1·02<br> +W. 1·47<br> +3·20</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">—— Etna, in volcanic +(Waltershausen)</td> +<td align="right" valign="top">40·91<br> +13·68<br> +nil<br> +17·49<br> +13·44<br> +13·19<br> +nil<br> +nil<br> +W. 0·85<br> +3·01</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">URALITE.<br> +—— Ural, (Rammelsberg)</td> +<td align="right" valign="top">50·75<br> +5·65<br> +nil<br> +17·27<br> +11·59<br> +12·28<br> +nil<br> +nil<br> +W. 1·80<br> +3·14</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">AUGITE.<br> +—— Bohemia, in dolerite (Rammelsberg)</td> +<td align="right" valign="top">51·12<br> +3·38<br> +0·95<br> +8·08<br> +23·54<br> +12·82<br> +nil<br> +nil<br> +nil<br> +3·35</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">—— Vesuvius, in lava +of 1858 (Rammelsberg)</td> +<td align="right" valign="top">49·61<br> +4·42<br> +nil<br> +9·08<br> +22·83<br> +14·22<br> +nil<br> +nil<br> +nil<br> +3·25</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">DIALLAGE.<br> +—— Harz, in Gabbro (Rammelsberg)</td> +<td align="right" valign="top">52·00<br> +3·10<br> +nil<br> +9·36<br> +16·29<br> +18·51<br> +nil<br> +nil<br> +W. 1·10<br> +3·23</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">HYPERSTHENE.<br> +—— Labrador, in Labrador-Rock (Damour)</td> +<td align="right" valign="top">51·36<br> +0·37<br> +nil<br> +22·59<br> +3·09<br> +21·31<br> +nil<br> +nil<br> +nil<br> +3·39</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="center" colspan="5">THE OLIVINE GROUP</td> +</tr> + +<tr> +<td align="left" valign="middle">BRONZITE.<br> +—— Greenland (V. Kobell)</td> +<td align="right" valign="top">58·00<br> +1·33<br> +11·14<br> +nil<br> +nil<br> +29·66<br> +nil<br> +nil<br> +nil<br> +3·20</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">OLIVINE.<br> +—— Carlsbad, in basalt (Rammelsberg)</td> +<td align="right" valign="top">39·34<br> +nil<br> +nil<br> +14·85<br> +nil<br> +45·81<br> +nil<br> +nil<br> +nil<br> +3·40</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> + +<tr> +<td align="left" valign="middle">—— Mount Somma, in +volcanic (Walmstedt)</td> +<td align="right" valign="top">10·08<br> +0·18<br> +nil<br> +15·74<br> +nil<br> +44·22<br> +nil<br> +nil<br> +nil<br> +3·33</td> +<td align="left" valign="top">Silica<br> +Alumina<br> +Sesquioxide of Iron<br> +Protoxides of Iron and Manganese<br> +Lime<br> +Magnesia<br> +Potash<br> +Soda<br> +Other constituents<br> +Specific gravity</td> +</tr> +</table> + +<p class="fnote2">In the “Other constituents” the +following signs are used: F=Fluorine, Li=Lithia, W=Loss on igniting +the mineral, in most instances only Water.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 500">[ 500 ]</a></p> + +<p>From the table above it will be observed that many minerals are +omitted which, even if they are of common occurrence, are more to +be regarded as accessory than as essential components of the rocks +in which they are found.* Such are, for example, Garnet, Epidote, +Tourmaline, Idocrase, Andalusite, Scapolite, the various Zeolites, +and several other silicates of somewhat rarer occurrence. +Magnetite, Titanoferrite, and Iron-pyrites also occur as normal +constituents of various igneous rocks, although in very small +amount, as also Apatite, or phosphate of lime. The other salts of +lime, including its carbonate or calcite, although often met with, +are invariably products of secondary chemical action.</p> + +<p>The Zeolites, above mentioned, so named from the manner in which +they froth up under the blow-pipe and melt into a glass, differ in +their chemical composition from all the other mineral constituents +of volcanic rocks, since they are hydrated silicates containing +from 10 to 25 per cent of water. They abound in some trappean rocks +and ancient lavas, where they fill up vesicular cavities and +interstices in the substance of the rocks, but are rarely found in +any quantity in recent lavas; in most cases they are to be regarded +as secondary products formed by the action of water on the other +constituents of the rocks. Among them the species Analcime, +Stilbite, Natrolite, and Chabazite may be mentioned as of most +common occurrence.</p> + +<p><b>Quartz Group.</b>—The microscope has shown that pure +quartz is oftener present in lavas than was formerly supposed. It +had been argued that the quartz in granite having a specific +gravity of 2·6, was not of purely igneous origin, because +the silica resulting from fusion in the laboratory has only a +specific gravity of 2·3. But Mr. David Forbes has +ascertained that the free quartz in trachytes, which are known to +have flowed as lava, has the same specific gravity as the ordinary +quartz of granite; and the recent researches of Von Rath and others +prove that the mineral Tridymite, which is crystallised silica of +specific gravity 2·3 (see Table, p. 499), is of common +occurrence in the volcanic rocks of Mexico, Auvergne, the Rhine, +and elsewhere, although hitherto entirely overlooked.</p> + +<p><b>Feldspar Group.</b>—In the Feldspar group (Table, p. +499) the five mineral species most commonly met with as rock +constituents are: 1. Orthoclase, often called common or +potash-feldspar. 2. Albite, or soda-feldspar, a mineral which plays +a more subordinate part than was formerly supposed, this name +having been given to much which has since been proved to be +Oligoclase. 3. Oligoclase, or soda-lime feldspar,</p> + +<p class="fnote">* For analyses of these minerals see the +Mineralogies of Dana and Bristow.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 501">[ 501 ]</a></p> + +<p>in which soda is present in much larger proportion than lime, +and of which mineral andesite are andesine, is considered to be a +variety. 4. Labradorite, or lime-soda-feldspar, in which the +proportions of lime and soda are the reverse to what they are in +Oligoclase. 5. Anorthite or lime-feldspar. The two latter feldspars +are rarely if ever found to enter into the composition of rocks +containing quartz.</p> + +<p>In employing such terms as potash-feldspar, etc., it must, +however, always be borne in mind that it is only intended to direct +attention to the predominant alkali or alkaline earth in the +mineral, not to assert the absence of the others, which in most +cases will be found to be present in minor quantity. Thus +potash-feldspar (orthoclase) almost always contains a little soda, +and often traces of lime or magnesia; and in like manner with the +others. The terms “glassy” and “compact” +feldspars only refer to structure, and not to species or +composition; the student should be prepared to meet with any of the +above feldspars in either of these conditions: the glassy state +being apparently due to quick cooling, and the compact to +conditions unfavourable to crystallisation; the so-called +“compact feldspar” is also very commonly found to be an +admixture of more than one feldspar species, and frequently also +contains quartz and other extraneous mineral matter only to be +detected by the microscope.</p> + +<p>Feldspars when arranged according to their system of +crystallisation are <i>monoclinic,</i> having one axis obliquely +inclined; or <i>triclinic,</i> having the three axes all obliquely +inclined to each other. If arranged with reference to their +cleavage they are <i>orthoclastic,</i> the fracture taking place +always at a right angle; or <i>plagioclastic,</i> in which the +cleavages are oblique to one another. Orthoclase is orthoclastic +and monoclinic; all the other feldspars are plagioclastic and +triclinic.</p> + +<p><i>Minerals in Meteorites.</i>—That variety of the +Feldspar Group which is called Anorthite has been shown by +Rammelsberg to occur in a meteoric stone, and his analysis proves +it to be almost identical in its chemical proportions to the same +mineral in the lavas of modern volcanoes. So also Bronzite +(Enstatite) and Olivine have been met with in meteorites shown by +analysis to come remarkably near to these minerals in ordinary +rocks.</p> + +<p><b>Mica Group.</b>—With regard to the micas, the four +principal species (Table, p. 499) all contain potash in nearly the +same proportion, but differ greatly in the proportion and nature of +their other ingredients. Muscovite is often called common or potash +mica; Lepidolite is characterised by containing lithia in addition; +Biotite contains a large amount of</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 502">[ 502 ]</a></p> + +<p>magnesia and oxide of iron; whilst Phlogopite contains still +more of the former substance. In rocks containing quartz, muscovite +or lepidolite are most common. The mica in recent volcanic rocks, +gabbros, and diorites is usually Biotite, while that so common in +metamorphic limestones is usually, if not always, Phlogopite.</p> + +<p><b>Amphibole and Pyroxene Group.</b>—The minerals included +in the table under the Amphibole and Pyroxene Group differ somewhat +in their crystallisation form, though they all belong to the +monoclinic system. Amphibole is a general name for all the +different varieties of Hornblende, Actinolite, Tremolite, etc., +while Pyroxene includes Augite, Diallage, Malacolite, Sahlite, etc. +The two divisions are so much allied in chemical composition and +crystallographic characters, and blend so completely one into the +other in Uralite (see <a href="#page 499">page 499</a>), that it is +perhaps best to unite them in one group.</p> + +<p><b>Theory of Isomorphism.</b>—The history of the changes +of opinion on this point is curious and instructive. Werner first +distinguished augite from hornblende; and his proposal to separate +them obtained afterwards the sanction of Haüy, Mohs, and other +celebrated mineralogists. It was agreed that the form of the +crystals of the two species was different, and also their +structure, as shown by <i>cleavage</i>—that is to say, by breaking +or cleaving the mineral with a chisel, or a blow of the hammer, in +the direction in which it yields most readily. It was also found by +analysis that augite usually contained more lime, less alumina, and +no fluoric acid; which last, though not always found in hornblende, +often enters into its composition in minute quantity. In addition +to these characters, it was remarked as a geological fact, that +augite and hornblende are very rarely associated together in the +same rock. It was also remarked that in the crystalline slags of +furnaces augitic forms were frequent, the hornblendic entirely +absent; hence it was conjectured that hornblende might be the +result of slow, and augite of rapid cooling. This view was +confirmed by the fact that Mitscherlich and Berthier were able to +make augite artificially, but could never succeed in forming +hornblende. Lastly, Gustavus Rose fused a mass of hornblende in a +porcelain furnace, and found that it did not, on cooling, assume +its previous shape, but invariably took that of augite. The same +mineralogist observed certain crystals called Uralite (see Table, +<a href="#page 499">p. 499</a>) in rocks from Siberia, which +possessed the cleavage and chemical composition of hornblende, +while they had the external form of augite.</p> + +<p>If, from these data, it is inferred that the same substance</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 503">[ 503 ]</a></p> + +<p>may assume the crystalline forms of hornblende or augite +indifferently, according to the more or less rapid cooling of the +melted mass, it is nevertheless certain that the variety commonly +called augite, and recognised by a peculiar crystalline form, has +usually more lime in it, and less alumina, than that called +hornblende, although the quantities of these elements do not seem +to be always the same. Unquestionably the facts and experiments +above mentioned show the very near affinity of hornblende and +augite; but even the convertibility of one into the other, by +melting and recrystallising, does not perhaps demonstrate their +absolute identity. For there is often some portion of the materials +in a crystal which are not in perfect chemical combination with the +rest. Carbonate of lime, for example, sometimes carries with it a +considerable quantity of silex into its own form of crystal, the +silex being mechanically mixed as sand, and yet not preventing the +carbonate of lime from assuming the form proper to it. This is an +extreme case, but in many others some one or more of the +ingredients in a crystal may be excluded from perfect chemical +union; and after fusion, when the mass recrystallises, the same +elements may combine perfectly or in new proportions, and thus a +new mineral may be produced. Or some one of the gaseous elements of +the atmosphere, the oxygen for example, may, when the melted matter +reconsolidates, combine with some one of the component +elements.</p> + +<p>The different quantity of the impurities or the refuse above +alluded to, which may occur in all but the most transparent and +perfect crystals, may partly explain the discordant results at +which experienced chemists have arrived in their analysis of the +same mineral. For the reader will often find that crystals of a +mineral determined to be the same by physical characters, +crystalline form, and optical properties, have been declared by +skilful analysers to be composed of distinct elements. This +disagreement seemed at first subversive of the atomic theory, or +the doctrine that there is a fixed and constant relation between +the crystalline form and structure of a mineral and its chemical +composition. The apparent anomaly, however, which threatened to +throw the whole science of mineralogy into confusion, was +reconciled to fixed principles by the discoveries of Professor +Mitscherlich at Berlin, who ascertained that the composition of the +minerals which had appeared so variable was governed by a general +law, to which he gave the name of <i>isomorphism</i> (from <i> +isos,</i> equal, and <i>morphe,</i> form). According to this law, +the ingredients of a given species of mineral are not</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 504">[ 504 ]</a></p> + +<p>absolutely fixed as to their kind and quality; but one +ingredient may be replaced by an equivalent portion of some +analogous ingredient. Thus, in augite, the lime may be in part +replaced by portions of protoxide of iron, or of manganese, while +the form of the crystal, and the angle of its cleavage planes, +remain the same. These vicarious substitutions, however, of +particular elements can not exceed certain defined limits.</p> + +<p><b>Basaltic Rocks.</b>—The two principal families of +trappean or volcanic rocks are the basalts and the trachytes, which +differ chiefly from each other in the quantity of silica which they +contain. The basaltic rocks are comparatively poor in silica, +containing less than 50 per cent of that mineral, and none in a +pure state or as free quartz, apart from the rest of the matrix. +They contain a larger proportion of lime and magnesia than the +trachytes, so that they are heavier, independently of the frequent +presence of the oxides of iron which in some cases forms more than +a fourth part of the whole mass. Abich has, therefore, proposed +that we should weigh these rocks, in order to appreciate their +composition in cases where it is impossible to separate their +component minerals. Thus, basalt from Staffa, containing +47·80 per cent of silica, has a specific gravity of +2·95; whereas trachyte, which has 66 per cent of silica, has +a specific gravity of only 2·68; trachytic porphyry, +containing 69 per cent of silica, a specific gravity of only +2·58. If we then take a rock of intermediate composition, +such as that prevailing in the Peak of Teneriffe, which Abich calls +Trachyte-dolerite, its proportion of silica being intermediate, or +58 per cent, it weighs 2·78, or more than trachyte, and less +than basalt.*</p> + +<p><i>Basalt.</i>—The different varieties of this rock are +distinguished by the names of basalts, anamezites, and dolerites, +names which, however, only denote differences in texture without +implying any difference in mineral or chemical composition: the +term <i>Basalt</i> being used only when the rock is compact, +amorphous, and often semi-vitreous in texture, and when it breaks +with a perfect conchoidal fracture; when, however, it is uniformly +crystalline in appearance, yet very close-grained, the name <i> +Anamesite</i> (from <i>anamesos,</i> intermediate) is employed, but +if the rock be so coarsely crystallised that its different mineral +constituents can be easily recognised by the eye, it is called <i> +Dolerite</i> (from <i>doleros,</i> deceitful), in allusion to the +difficulty of distinguishing it from some of the rocks known as +Plutonic.</p> + +<p><i>Melaphyre</i> is often quite undistinguishable in +external</p> + +<p class="fnote">* Dr. Daubeny on Volcanoes, 2nd ed., pp. 14, +15.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 505">[ 505 ]</a></p> + +<p>appearance from basalt, for although rarely so heavy, +dark-coloured, or compact, it may present at times all these +varieties of texture. Both these rocks are composed of triclinic +feldspar and augite with more or less olivine, magnetic or +titaniferous oxide of iron, and usually a little nepheline, +leucite, and apatite; basalt usually contains considerably more +olivine than melaphyre, but chemically they are closely allied, +although the melaphyres usually contain more silica and alumina, +with less oxides of iron, lime, and magnesia, than the basalts. The +Rowley Hills in Staffordshire, commonly known as Rowley Ragstone, +are melaphyre.</p> + +<p><i>Greenstone.</i>—This name has usually been extended to +all granular mixtures, whether of hornblende and feldspar, or of +augite and feldspar. The term <i>diorite</i> has been applied +exclusively to compounds of hornblende and triclinic feldspar. <i> +Labrador-rock</i> is a term used for a compound of labradorite or +labrador-feldspar and hypersthene; when the hypersthene +predominates it is sometimes known under the name of <i> +Hypersthene-rock.</i> <i>Gabbro</i> and <i>Diabase</i> are rocks +mainly composed of triclinic feldspars and diallage. All these +rocks become sometimes very crystalline, and help to connect the +volcanic with the Plutonic formations, which will be treated of in +Chapter XXXI.</p> + +<p><b>Trachytic Rocks.</b>—The name trachyte (from <i> +trachus,</i> rough) was originally given to a coarse granular +feldspathic rock which was rough and gritty to the touch. The term +was subsequently made to include other rocks, such as clinkstone +and obsidian, which have the same mineral composition, but to +which, owing to their different texture, the word in its original +meaning would not apply. The feldspars which occur in Trachytic +rocks are invariably those which contain the largest proportion of +silica, or from 60 to 70 per cent of that mineral. Through the base +are usually disseminated crystals of glassy feldspar, mica, and +sometimes hornblende. Although quartz is not a necessary ingredient +in the composition of this rock, it is very frequently present, and +the quartz trachytes are very largely developed in many volcanic +districts. In this respect the trachytes differ entirely from the +members of the Basaltic family, and are more nearly allied to the +granites.</p> + +<p><i>Obsidian.</i>—Obsidian, Pitchstone, and Pearlstone are +only different forms of a volcanic glass produced by the fusion of +trachytic rocks. The distinction between them is caused by +different rates of cooling from the melted state, as has been +proved by experiment. Obsidian is of a black or ash-grey colour, +and though opaque in mass is transparent in thin edges.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 506">[ 506 ]</a></p> + +<p><i>Clinkstone or Phonolite.</i>—Among the rocks of the +trachytic family, or those in which the feldspars are rich in +silica, that termed Clinkstone or Phonolite is conspicuous by its +fissile structure, and its tendency to lamination, which is such as +sometimes to render it useful as roofing-slate. It rings when +struck with the hammer, whence its name; is compact, and usually of +a greyish blue or brownish colour; is variable in composition, but +almost entirely composed of feldspar. When it contains disseminated +crystals of feldspar, it is called <i>Clinkstone porphyry.</i></p> + +<p><b>Volcanic Rocks distinguished by special Forms of +Structure.</b>—Many volcanic rocks are commonly spoken of +under names denoting structure alone, which must not be taken to +imply that they are distinct rocks, i.e., that they differ from one +another either in mineral or chemical composition. Thus the terms +Trachytic porphyry, Trachytic tuff, etc., merely refer to the same +rock under different conditions of mechanical aggregation or +crystalline development which would be more correctly expressed by +the use of the adjective, as porphyritic trachyte, etc., but as +these terms are so commonly employed it is considered advisable to +direct the student’s attention to them.</p> + +<img src="../images4/fig586.jpg" width="179" height="232" alt= +"Fig. 586: Porphyry. White crystals of feldspar in a dark base of hornblende and feldspar." + align="left"> + +<p><i>Porphyry</i> is one of this class, and very characteristic of +the volcanic formations. When distinct crystals of one or more +minerals are scattered through an earthy or compact base, the rock +is termed a porphyry (see Fig. 586). Thus trachyte is usually +porphyritic; for in it, as in many modern lavas, there are crystals +of feldspar; but in some porphyries the crystals are of augite, +olivine, or other minerals. If the base be greenstone, basalt, or +pitchstone, the rock may be denominated greenstone-porphyry, +pitchstone-porphyry, and so forth. The old classical type of this +form of rock is the red porphyry of Egypt, or the well-known +“Rosso antico.” It consists, according to Delesse, of a +red feldspathic base in which are disseminated rose-coloured +crystals of the feldspar called oligoclase, with some plates of +blackish hornblende and grains of oxide of iron (iron-glance). <i> +Red quartziferous porphyry</i> is a much more siliceous rock, +containing about 70 or 80 per cent of silex, while that of Egypt +has only 62 per cent.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 507">[ 507 ]</a></p> + +<p><i>Amygdaloid.</i>—This is also another form of igneous +rock, admitting of every variety of composition. It comprehends any +rock in which round or almond-shaped nodules of some mineral, such +as agate, chalcedony, calcareous spar, or zeolite, are scattered +through a base of wacke, basalt, greenstone, or other kind of trap. +It derives its name from the Greek word <i>amygdalon,</i> an +almond. The origin of this structure can not be doubted, for we may +trace the process of its formation in modern lavas. Small pores or +cells are caused by bubbles of steam and gas confined in the melted +matter. After or during consolidation, these empty spaces are +gradually filled up by matter separating from the mass, or +infiltered by water permeating the rock. As these bubbles have been +sometimes lengthened by the flow of the lava before it finally +cooled, the contents of such cavities have the form of almonds. In +some of the amygdaloidal traps of Scotland, where the nodules have +decomposed, the empty cells are seen to have a glazed or vitreous +coating, and in this respect exactly resemble scoriaceous lavas, or +the slags of furnaces.</p> + +<img src="../images4/fig587.jpg" width="206" height="259" alt= +"Fig. 587: Scoriaceous lava in part converted into an amygdaloid." +align="right"> + +<p>Fig. 587 represents a fragment of stone taken from the upper +part of a sheet of basaltic lava in Auvergne. One-half is +scoriaceous, the pores being perfectly empty; the other part is +amygdaloidal, the pores or cells being mostly filled up with +carbonate of lime, forming white kernels.</p> + +<p><i>Lava.</i>—This term has a somewhat vague signification, +having been applied to all melted matter observed to flow in +streams from volcanic vents. When this matter consolidates in the +open air, the upper part is usually scoriaceous, and the mass +becomes more and more stony as we descend, or in proportion as it +has consolidated more slowly and under greater pressure. At the +bottom, however, of a stream of lava, a small portion of +scoriaceous rock very frequently occurs, formed by the first thin +sheet of liquid matter, which often precedes the main current, and +solidifies under slight pressure.</p> + +<p>The more compact lavas are often porphyritic, but even the +scoriaceous part sometimes contains imperfect crystals, which have +been derived from some older rocks, in which</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 508">[ 508 ]</a></p> + +<p>the crystals pre-existed, but were not melted, as being more +infusible in their nature. Although melted matter rising in a +crater, and even that which enters a rent on the side of a crater, +is called lava, yet this term belongs more properly to that which +has flowed either in the open air or on the bed of a lake or sea. +If the same fluid has not reached the surface, but has been merely +injected into fissures below ground, it is called trap. There is +every variety of composition in lavas; some are trachytic, as in +the Peak of Teneriffe; a great number are basaltic, as in Vesuvius +and Auvergne; others are andesitic, as those of Chili; some of the +most modern in Vesuvius consist of green augite, and many of those +of Etna of augite and labrador-feldspar.*</p> + +<p><i>Scoriæ</i> and <i>Pumice</i> may next be mentioned, as +porous rocks produced by the action of gases on materials melted by +volcanic heat. <i>Scoriæ</i> are usually of a reddish-brown +and black colour, and are the cinders and slags of basaltic or +augitic lavas. <i>Pumice</i> is a light, spongy, fibrous substance, +produced by the action of gases on trachytic and other lavas; the +relation, however, of its origin to the composition of lava is not +yet well understood. Von Buch says that it never occurs where only +labrador-feldspar is present.</p> + +<p><i>Volcanic Ash or Tuff, Trap Tuff.</i>—Small angular +fragments of the scoriæ and pumice, above-mentioned, and the +dust of the same, produced by volcanic explosions, form the tuffs +which abound in all regions of active volcanoes, where showers of +these materials, together with small pieces of other rocks ejected +from the crater, and more or less burnt, fall down upon the land or +into the sea. Here they often become mingled with shells, and are +stratified. Such tuffs are sometimes bound together by a calcareous +cement, and form a stone susceptible of a beautiful polish. But +even when little or no lime is present, there is a great tendency +in the materials of ordinary tuffs to cohere together. The term <i> +volcanic ash</i> has been much used for rocks of all ages supposed +to have been derived from matter ejected in a melted state from +volcanic orifices. We meet occasionally with extremely compact beds +of volcanic materials, interstratified with fossiliferous rocks. +These may sometimes be tuffs, although their density or compactness +is such as the cause them to resemble many of those kinds of trap +which are found in ordinary dikes.</p> + +<p><i>Wacke</i> is a name given to a decomposed state of various +trap rocks of the basaltic family, or those which are poor in +silica. It resembles clay of a yellowish or brown colour, and</p> + +<p class="fnote">* G. Hose, Ann. des Mines, tome viii, p. 32.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 509">[ 509 ]</a></p> + +<p>passes gradually from the soft state to the hard dolerite, +greenstone, or other trap rock from which it has been derived.</p> + +<p><i>Agglomerate.</i>—In the neighbourhood of volcanic +vents, we frequently observe accumulations of angular fragments of +rocks formed during eruptions by the explosive action of steam, +which shatters the subjacent stony formations, and hurls them up +into the air. They then fall in showers around the cone or crater, +or may be spread for some distance over the surrounding country. +The fragments consist usually of different varieties of scoriaceous +and compact lavas; but other kinds of rock, such as granite or even +fossiliferous limestones, may be intermixed; in short, any +substance through which the expansive gases have forced their way. +The dispersion of such materials may be aided by the wind, as it +varies in direction or intensity, and by the slope of the cone down +which they roll, or by floods of rain, which often accompany +eruptions. But if the power of running water, or of the waves and +currents of the sea, be sufficient to carry the fragments to a +distance, it can scarcely fail to wear off their angles, and the +formation then becomes a <i>conglomerate.</i> If occasionally +globular pieces of scoriæ abound in an agglomerate, they may +not owe their round form to attrition. When all the angular +fragments are of volcanic rocks the mass is usually termed a +volcanic breccia.</p> + +<p><i>Laterite</i> is a red or brick-like rock composed of silicate +of alumina and oxide of iron. The red layers called “ochre +beds,” dividing the lavas of the Giant’s Causeway, are +laterites. These were found by Delesse to be trap impregnated with +the red oxide of iron, and in part reduced to kaolin. When still +more decomposed, they were found to be clay coloured by red ochre. +As two of the lavas of the Giant’s Causeway are parted by a +bed of lignite, it is not improbable that the layers of laterite +seen in the Antrim cliffs resulted from atmospheric decomposition. +In Madeira and the Canary Islands streams of lava of subaërial +origin are often divided by red bands of laterite, probably ancient +soils formed by the decomposition of the surfaces of lava-currents, +many of these soils having been coloured red in the atmosphere by +oxide of iron, others burnt into a red brick by the overflowing of +heated lavas. These red bands are sometimes prismatic, the small +prisms being at right angles to the sheets of lava. Red clay or red +marl, formed as above stated by the disintegration of lava, +scoriæ, or tuff, has often accumulated to a great thickness +in the valleys of Madeira, being washed into them by alluvial +action; and some of the thick beds of</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 510">[ 510 ]</a></p> + +<p>laterite in India may have had a similar origin. In India, +however, especially in the Deccan, the term “laterite” +seems to have been used too vaguely to answer the above definition. +The vegetable soil in the gardens of the suburbs of Catania which +was overflowed by the lava of 1669 was turned or burnt into a layer +of red brick-coloured stone, or in other words, into laterite, +which may now be seen supporting the old lava-current.</p> + +<p><b>Columnar and Globular Structure.</b>—One of the +characteristic forms of volcanic rocks, especially of basalt, is +the columnar, where large masses are divided into regular prisms, +sometimes easily separable, but in other cases adhering firmly +together. The columns vary, in the number of angles, from three to +twelve; but they have most commonly from five to seven sides. They +are often divided transversely, at nearly equal distances, like the +joints in a vertebral column, as in the Giant’s Causeway, in +Ireland. They vary exceedingly in respect to length and diameter. +Dr. MacCulloch mentions some in Skye which are about 400 feet long; +others, in Morven, not exceeding an inch. In regard to diameter, +those of Ailsa measure nine feet, and those of Morven an inch or +less.* They are usually straight, but sometimes curved; and +examples of both these occur in the island of Staffa. In a +horizontal bed or sheet of trap the columns are vertical; in a +vertical dike they are horizontal.</p> + +<center><img src="../images4/fig588.jpg" width="332" height="181" alt= +"Fig. 588: Lava of La Coupe d'Ayzac, near Antraigue, in the Department of Ardêche."> +</center> + +<p>It being assumed that columnar trap has consolidated from a +fluid state, the prisms are said to be always at right angles to +the <i>cooling surfaces.</i> If these surfaces, therefore, instead +of being either perpendicular or horizontal, are curved, the +columns ought to be inclined at every angle to the horizon; and +there is a beautiful exemplification of this phenomenon in one of +the valleys of the Vivarais, a mountainous</p> + +<p class="fnote">* MacCulloch Sys. of Geol., vol. ii, p. 137.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 511">[ 511 ]</a></p> + +<p>district in the South of France, where, in the midst of a region +of gneiss, a geologist encounters unexpectedly several volcanic +cones of loose sand and scoriæ. From the crater of one of +these cones, called La Coupe d’Ayzac, a stream of lava has +descended and occupied the bottom of a narrow valley, except at +those points where the river Volant, or the torrents which join it, +have cut away portions of the solid lava. Fig. 588 represents the +remnant of the lava at one of these points. It is clear that the +lava once filled the whole valley up to the dotted line <i>d a</i>; +but the river has gradually swept away all below that line, while +the tributary torrent has laid open a transverse section; by which +we perceive, in the first place, that the lava is composed, as +usual in this country, of three parts: the uppermost, at <i>a,</i> +being scoriaceous, the second <i>b,</i> presenting irregular +prisms; and the third, <i>c,</i> with regular columns, which are +vertical on the banks of the Volant, where they rest on a +horizontal base of gneiss, but which are inclined at an angle of +45°, at <i>g,</i> and are nearly horizontal at <i>f,</i> their +position having been everywhere determined, according to the law +before mentioned, by the form of the original valley.</p> + +<img src="../images4/fig589.jpg" width="172" height="251" alt= +"Fig. 589: Columnar basalt in the Vicentin." align="right"> + +<p>In Fig. 589, a view is given of some of the inclined and curved +columns which present themselves on the sides of the valleys in the +hilly region north of Vicenza, in Italy, and at the foot of the +higher Alps.* Unlike those of the Vivarais, last mentioned, the +basalt of this country was evidently submarine, and the present +valleys have since been hollowed out by denudation.</p> + +<p>The columnar structure is by no means peculiar to the trap rocks +in which augite abounds; it is also observed in trachyte, and other +feldspathic rocks of the igneous class, although in these it is +rarely exhibited in such regular polygonal forms. It has been +already stated that basaltic columns are often divided by +cross-joints. Sometimes each segment, instead of an angular, +assumes a spheroidal form, so that a pillar is made up of a pile of +balls, usually flattened, as in the Cheese-grotto at +Bertrich-Baden, in the Eifel, near the Moselle (Fig. 590). The +basalt there is part of a small</p> + +<p class="fnote">* Fortis, Mém. sur l’Hist. Nat. de +l’Italie, tome 1., p. 233, plate 7.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 512">[ 512 ]</a></p> + +<img src="../images4/fig590.jpg" width="281" height="269" alt= +"Fig. 590: Basaltic pillars of Käsegrotte, Bertrich-Baden, half-way between Trèves and Coblenz." + align="left"> + +<p>stream of lava, from 30 to 40 feet thick, which has proceeded +from one of several volcanic craters, still extant, on the +neighbouring heights.</p> + +<p>In some masses of decomposing greenstone, basalt, and other trap +rocks, the globular structure is so conspicuous that the rock has +the appearance of a heap of large cannon balls. According to M. +Delesse, the centre of each spheroid has been a centre of +crystallisation, around which the different minerals of the rock +arranged themselves symmetrically during the process of cooling. +But it was also, he says, a centre of contraction, produced by the +same cooling, the globular form, therefore, of such spheroids being +the combined result of crystallisation and contraction.*</p> + +<img src="../images4/fig591.jpg" width="172" height="337" alt= +"Fig. 591: Globiform pitchstone. Chiaja di Luna, Isle of Ponza." +align="right"> + +<p>Mr. Scrope gives as an illustration of this structure a resinous +trachyte or pitchstone-porphyry in one of the Ponza islands, which +rise from the Mediterranean, off the coast of Terracina and Gaeta. +The globes vary from a few inches to three feet in diameter, and +are of an ellipsoidal form (see Fig. 591). The whole rock is in a +state of decomposition, “and when the balls,” says Mr. +Scrope, “have been exposed a short time to the weather, they +scale off at a touch into numerous concentric coats, like those of +a bulbous root, inclosing a compact nucleus. The laminæ</p> + +<p class="fnote">* Delesse, sur les Roches Globuleuses, Mém. +de la Soc. Géol. de France, 2 sér., tome iv.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 513">[ 513 ]</a></p> + +<p>of this nucleus have not been so much loosened by decomposition; +but the application of a ruder blow will produce a still further +exfoliation.”*</p> + +<img src="../images4/fig592.jpg" width="243" height="226" alt= +"Fig. 592: Dike in valley, near Brazen Head, Madeira. (From a drawing of Captain Basil Hall, R.N.)" + align="right"> + +<p><b>Volcanic or Trap Dikes.</b>—The leading varieties of +the trappean rocks—basalt, greenstone, trachyte, and the rest—are +found sometimes in dikes penetrating stratified and unstratified +formations, sometimes in shapeless masses protruding through or +overlying them, or in horizontal sheets intercalated between +strata. Fissures have already been spoken of as occurring in all +kinds of rocks, some a few feet, others many yards in width, and +often filled up with earth or angular pieces of stone, or with sand +and pebbles. Instead of such materials, suppose a quantity of +melted stone to be driven or injected into an open rent, and there +consolidated, we have then a tabular mass resembling a wall, and +called a trap dike. It is not uncommon to find such dikes passing +through strata of soft materials, such as tuff, scoriæ, or +shale, which, being more perishable than the trap, are often washed +away by the sea, rivers, or rain, in which case the dike stands +prominently out in the face of precipices, or on the level surface +of a country (see Fig. 592).</p> + +<p>In the islands of Arran and Skye, and in other parts of +Scotland, where sandstone, conglomerate, and other hard rocks are +traversed by dikes of trap, the converse of the above phenomenon is +seen. The dike, having decomposed more rapidly than the containing +rock, has once more left open the original fissure, often for a +distance of many yards inland from the sea-coast. There is yet +another case, by no means uncommon in Arran and other parts of +Scotland, where the strata in contact with the dike, and for a +certain distance from it, have been hardened, so as to resist the +action of the weather more than the dike itself, or the surrounding +rocks. When this happens, two parallel walls of indurated strata +are seen protruding above the general level of the country and +following the course of the dike. In Fig. 593, a ground plan is +given of a ramifying dike of greenstone,</p> + +<p class="fnote">* Scrope, Geol. Trans., 2nd series, vol. ii, p. +205.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 514">[ 514 ]</a></p> + +<p>which I observed cutting through sandstone on the beach near +Kildonan Castle, in Arran. The larger branch varies from five to +seven feet in width, which will afford a scale of measurement for +the whole.</p> + +<center><img src="../images4/fig593.jpg" width="303" height="148" alt= +"Fig. 593: Ground-plan of greenstone dikes traversing sandstone."> +</center> + +<p>In the Hebrides and other countries, the same masses of trap +which occupy the surface of the country far and wide, concealing +the subjacent stratified rocks, are seen also in the sea-cliffs, +prolonged downward in veins or dikes, which probably unite with +other masses of igneous rock at a greater depth. The largest of the +dikes represented in Fig. 594, and which are seen in part of the +coast of Skye, is no less than 100 feet in width.</p> + +<center><img src="../images4/fig594.jpg" width="329" height="101" alt= +"Fig. 594: Trap dividing and covering sandstone near Suishnish, in Skye."> +</center> + +<p>Every variety of trap-rock is sometimes found in dikes, as +basalt, greenstone, feldspar-porphyry, and trachyte. The +amygdaloidal traps also occur, though more rarely, and even tuff +and breccia, for the materials of these last may be washed down +into open fissures at the bottom of the sea, or during eruption on +the land may be showered into them from the air. Some dikes of trap +may be followed for leagues uninterruptedly in nearly a straight +direction, as in the north of England, showing that the fissures +which they fill must have been of extraordinary length.</p> + +<p><b>Rocks altered by Volcanic Dikes.</b>—After these +remarks on the form and composition of dikes themselves, I shall +describe the alterations which they sometimes produce in the rocks +in contact with them. The changes are usually such as the heat of +melted matter and of the entangled steam and gases might be +expected to cause.</p> + +<p><i>Plas-Newydd: Dike cutting through Shale.</i>—A striking +example,</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 515">[ 515 ]</a></p> + +<p>near Plas-Newydd, in Anglesea, has been described by Professor +Henslow.* The dike is 134 feet wide, and consists of a rock which +is a compound of feldspar and augite (dolerite of some authors). +Strata of shale and argillaceous limestone, through which it cuts +perpendicularly, are altered to a distance of 30, or even, in some +places, of 35 feet from the edge of the dike. The shale, as it +approaches the trap, becomes gradually more compact, and is most +indurated where nearest the junction. Here it loses part of its +schistose structure, but the separation into parallel layers is +still discernible. In several places the shale is converted into +hard porcelanous jasper. In the most hardened part of the mass the +fossil shells, principally <i>Producti,</i> are nearly obliterated; +yet even here their impressions may frequently be traced. The +argillaceous limestone undergoes analogous mutations, losing its +earthy texture as it approaches the dike, and becoming granular and +crystalline. But the most extraordinary phenomenon is the +appearance in the shale of numerous crystals of analcime and +garnet, which are distinctly confined to those portions of the rock +affected by the dike.† Some garnets contain as much as 20 +per cent of lime, which they may have derived from the +decomposition of the fossil shells or <i>Producti.</i> The same +mineral has been observed, under very analogous circumstances, in +High Teesdale, by Professor Sedgwick, where it also occurs in shale +and limestone, altered by basalt.‡</p> + +<p><i>Antrim: Dike cutting through Chalk.</i>—In several +parts of the county of Antrim, in the north of Ireland, chalk with +flints is traversed by basaltic dikes. The chalk is there converted +into granular marble near the basalt, the change sometimes +extending eight or ten feet from the wall of the dike, being +greatest near the point of contact, and thence gradually decreasing +till it becomes evanescent. “The extreme effect,” says +Dr. Berger, “presents a dark brown crystalline limestone, the +crystals running in flakes as large as those of coarse primitive +(<i>metamorphic</i>) limestone; the next state is saccharine, then +fine grained and arenaceous; a compact variety, having a +porcelanous aspect and a bluish-grey colour, succeeds: this, +towards the outer edge, becomes yellowish-white, and insensibly +graduates into the unaltered chalk. The flints in the altered chalk +usually assume a grey yellowish colour.”§ All traces of +organic remains are effaced in that part of the limestone which is +most crystalline.</p> + +<p class="fnote">* Cambridge Transactions, vol. i, p. 402.<br> +† Ibid., vol. i, p. 410.<br> + ‡ Ibid., vol. ii, p. 175.<br> +§ Dr. Berger, Geol. Trans., 1st series, vol. iii, p. 172.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 516">[ 516 ]</a></p> + +<center><img src="../images4/fig595.jpg" width="337" height="147" alt= +"Fig. 595: Basaltic dikes in chalk in Island of Rathlin, Antrim. Ground-plan as seen on the beach."> +</center> + +<p>Fig. 595 represents three basaltic dikes traversing the chalk, +all within the distance of 90 feet. The chalk contiguous to the two +outer dikes is converted into a finely granular marble, <i>m, +m,</i> as are the whole of the masses between the outer dikes and +the central one. The entire contrast in the composition and colour +of the intrusive and invaded rocks, in these cases, renders the +phenomena peculiarly clear and interesting. Another of the dikes of +the north-east of Ireland has converted a mass of red sandstone +into hornstone. By another, the shale of the coal-measures has been +indurated, assuming the character of flinty slate; and in another +place the slate-clay of the lias has been changed into flinty +slate, which still retains numerous impressions of +ammonites.†</p> + +<p>It might have been anticipated that beds of coal would, from +their combustible nature, be affected in an extraordinary degree by +the contact of melted rock. Accordingly, one of the greenstone +dikes of Antrim, on passing through a bed of coal, reduces it to a +cinder for the space of nine feet on each side. At Cockfield Fell, +in the north of England, a similar change is observed. Specimens +taken at the distance of about thirty yards from the trap are not +distinguishable from ordinary pit-coal; those nearer the dike are +like cinders, and have all the character of coke; while those close +to it are converted into a substance resembling soot.‡</p> + +<p>It is by no means uncommon to meet with the same rocks, even in +the same districts, absolutely unchanged in the proximity of +volcanic dikes. This great inequality in the effects of the igneous +rocks may often arise from an original difference in their +temperature, and in that of the entangled gases, such as is +ascertained to prevail in different lavas, or in the same lava near +its source and at a distance from it. The power also of the invaded +rocks to conduct heat may vary,</p> + +<p class="fnote">* Geol. Trans., 1st series, vol. iii, p. 210 and +plate 10.<br> +† Ibid., vol. iii, p. 213; and Playfair, Illus. of Hutt. +Theory, s. 253.<br> +‡ Sedgwick, Camb. Trans., vol. ii, p. 37.)</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 517">[ 517 ]</a></p> + +<p>according to their composition, structure, and the fractures +which they may have experienced, and perhaps, also, according to +the quantity of water (so capable of being heated) which they +contain. It must happen in some cases that the component materials +are mixed in such proportions as to prepare them readily to enter +into chemical union, and form new minerals; while in other cases +the mass may be more homogeneous, or the proportions less adapted +for such union.</p> + +<p>We must also take into consideration, that one fissure may be +simply filled with lava, which may begin to cool from the first; +whereas in other cases the fissure may give passage to a current of +melted matter, which may ascend for days or months, feeding streams +which are overflowing the country above, or being ejected in the +shape of scoriæ from some crater. If the walls of a rent, +moreover, are heated by hot vapour before the lava rises, as we +know may happen on the flanks of a volcano, the additional heat +supplied by the dike and its gases will act more powerfully.</p> + +<p><b>Intrusion of Trap between Strata.</b>—Masses of trap +are not unfrequently met with intercalated between strata, and +maintaining their parallelism to the planes of stratification +throughout large areas. They must in some places have forced their +way laterally between the divisions of the strata, a direction in +which there would be the least resistance to an advancing fluid, if +no vertical rents communicated with the surface, and a powerful +hydrostatic pressure were caused by gases propelling the lava +upward.</p> + +<p><b>Relation of Trappean Rocks to the Products of active +Volcanoes.</b>—When we reflect on the changes above described +in the strata near their contact with trap dikes, and consider how +complete is the analogy or often identity in composition and +structure of the rocks called trappean and the lavas of active +volcanoes, it seems difficult at first to understand how so much +doubt could have prevailed for half a century as to whether trap +was of igneous or aqueous origin. To a certain extent, however, +there was a real distinction between the trappean formations and +those to which the term volcanic was almost exclusively confined. A +large portion of the trappean rocks first studied in the north of +Germany, and in Norway, France, Scotland, and other countries, were +such as had been formed entirely under water, or had been injected +into fissures and intruded between strata, and which had never +flowed out in the air, or over the bottom of a shallow sea. When +these products, therefore, of submarine or subterranean igneous +action were contrasted with loose cones of scoriæ, tuff, and +lava, or with narrow streams of lava in</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 518">[ 518 ]</a></p> + +<p>great part scoriaceous and porous, such as were observed to have +proceeded from Vesuvius and Etna, the resemblance seemed remote and +equivocal. It was, in truth, like comparing the roots of a tree +with its leaves and branches, which, although the belong to the +same plant, differ in form, texture, colour, mode of growth, and +position. The external cone, with its loose ashes and porous lava, +may be likened to the light foliage and branches, and the rocks +concealed far below, to the roots. But it is not enough to say of +the volcano,</p> + +<pre> + “Quantum vertice in auras + Ætherias, tantum radice in Tartara tendit,” +</pre> + +<p>for its roots do literally reach downward to Tartarus, or to the +regions of subterranean fire; and what is concealed far below is +probably always more important in volume and extent than what is +visible above ground.</p> + +<img src="../images4/fig596.jpg" width="171" height="164" alt= +"Fig. 596: Strata intercepted by a trap dike, and covered with alluvium." + align="left"> + +<p>We have already stated how frequently dense masses of strata +have been removed by denudation from wide areas (see <a href= +"ch6.html">Chapter VI</a>); and this fact prepares us to expect a +similar destruction of whatever may once have formed the uppermost +part of ancient submarine or subaërial volcanoes, more +especially as those superficial parts are always of the lightest +and most perishable materials. The abrupt manner in which dikes of +trap usually terminate at the surface (see Fig. 596), and the +water-worn pebbles of trap in the alluvium which covers the dike, +prove incontestably that whatever was uppermost in these formations +has been swept away. It is easy, therefore, to conceive that what +is gone in regions of trap may have corresponded to what is now +visible in active volcanoes.</p> + +<p>As to the absence of porosity in the trappean formations, the +appearances are in a great degree deceptive, for all amygdaloids +are, as already explained, porous rocks, into the cells of which +mineral matter such as silex, carbonate of lime, and other +ingredients, have been subsequently introduced (see <a href= +"#page 507">p. 507</a>); sometimes, perhaps, by secretion during +the cooling and consolidation of lavas. In the Little Cumbray, one +of the Western Islands, near Arran, the amygdaloid sometimes +contains elongated cavities filled with brown spar; and when the +nodules have been washed out, the</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 519">[ 519 ]</a></p> + +<p>interior of the cavities is glazed with the vitreous varnish so +characteristic of the pores of slaggy lavas. Even in some parts of +this rock which are excluded from air and water, the cells are +empty, and seem to have always remained in this state, and are +therefore undistinguishable from some modern lavas.*</p> + +<p>Dr. MacCulloch, after examining with great attention these and +the other igneous rocks of Scotland, observes, “that it is a +mere dispute about terms, to refuse to the ancient eruptions of +trap the name of submarine volcanoes; for they are such in every +essential point, although they no longer eject fire and +smoke.” The same author also considers it not improbable that +some of the volcanic rocks of the same country may have been poured +out in the open air.†</p> + +<p>It will be seen in the following chapters that in the +earth’s crust there are volcanic tuffs of all ages, +containing marine shells, which bear witness to eruptions at many +successive geological periods. These tuffs, and the associated +trappean rocks, must not be compared to lava and scoriæ which +had cooled in the open air. Their counterparts must be sought in +the products of modern submarine volcanic eruptions. If it be +objected that we have no opportunity of studying these last, it may +be answered, that subterranean movements have caused, almost +everywhere in regions of active volcanoes, great changes in the +relative level of land and sea, in times comparatively modern, so +as to expose to view the effects of volcanic operations at the +bottom of the sea.</p> + +<p class="fnote">* MacCulloch, West. Islands, vol. ii, p. 487.<br> +† Syst. of Geol., vol. ii, p. 114.</p> + +<br> +<hr> +<small><a href="contents.html">Contents</a> / <a href="ch27.html"> +Chapter XXVII</a> / <a href="ch29.html">Chapter XXIX</a></small> +</body> +</html> + |